1. General Background
This invention relates generally to a medication infusion device for administering fluid to patients and more particularly to an improved, ambulatory infusion device with a disposable administration set which is inexpensive to manufacture, convenient to operate and which ensues fluid delivery at a consistent and uniform rate. More specifically, this invention relates to an occlusion detection system for sensing a blockage in either a supply tube which provides medication to such an ambulatory infusion device or an outlet tube which provides medication from such an infusion device to a patient.
2. Description of the Prior Art
As a result of the ongoing need for improved health care, there is a continuous effort to improve the administration of intravenous fluid to patients. As is well known, medication dispensers and infusion devices are used for infusion of predetermined amounts of medication into the body of a patient. Various types of medication dispensers employing different techniques for a variety of applications are known to exist.
In many cases it is of critical importance to provide precisely controlled and consistent flow rates of intravenous fluid to patients. This need for more controlled IV flow rates is only partially fulfilled by prior art displacement pumps. Specifically, particularly when such pumps are intended for ambulatory use by a patient beyond the vicinity of a hospital or other health care facility, the occurrence of an occlusion in the pump's medication supply tube or output tube may endanger the patient without warning. If, for example, the supply reservoir is empty, or the supply tube becomes kinked, pinched or otherwise blocked, the supply of medication to the patient will cease. As the continued supply of some medications is necessary to sustain the patient or remedy the patient's condition, cessation of supply may even be life threatening. Yet, with most ambulatory pumps, such an occlusion would go unnoticed unless the patient is extraordinarily vigilant.
Similarly, if the needle or catheter which supplies the pump's output to the patient becomes blocked, or the outlet tubing from the pump becomes kinked or blocked, flow of medication to the patient will cease. Furthermore, with such blockage, it may be possible for dangerously high pressures to build within the outlet tube. As this tube is resilient, it may expand with the increased pressure, storing a significant volume of medication. If the rising pressure finally overcomes the blockage, the stored, pressurized medication may be supplied in a surge to the patient, overdosing and possibly endangering the patient.
As is well known, disposable equipment for medical applications is desirable so as to maintain a germ-free environment to prevent the transfer of infection especially where cost prohibits cleaning and sterilization after each use. Disposable elements, especially in an ambulatory infusion environment, must be low in cost, since clinical application of disposable administration sets requires that the administration sets be regularly replaced. Typically, such sets are replaced every 24 to 48 hours, and seldom remain in use longer than one week. This frequent replacement interval should ideally be fulfilled by an inexpensively molded, disposable, mechanism which would normally not last the years of service life expected from the pump itself. To be practical, an occlusion detection system must be able to reliably operate with such disposable administration sets. In the prior art, occlusion sensors have typically mechanically sensed the tubing of the administration set. Thus, such systems required a highly precise, repeatable positioning of the occlusion sensor against each new administration set. This requirement subjected prior art sensors to frequent failure and maintenance problems.
Furthermore, it is desirable to have a disposable administration set which is easy to load and unload to minimize operator errors. These factors can be very important in a clinical situation when a few extra seconds may be critical to a patient's life. Typically, prior art devices require several steps to accomplish the task of loading and unloading, particularly where a tube sensor must be placed against the administration set to detect occlusions.
Some prior art devices incorporate a pressure transducer and diaphragm assembly to monitor fluid pressure as an indication of occlusion. Such an occlusion detection technique is undesirable in view of the complexities and cost involved.
Still other prior art devices use strain gages to measure the diameter of the supply tube as a means of sensing occlusions. For example, it is known to sense the diameter of a supply tube with a strain gage to detect upstream or supply occlusions. Thus, upon the occurrence of an occlusions in the supply tube, or an empty supply bag or vessel, a vacuum will be drawn in the supply tube by the continued operation of the pump. Because the supply tube is formed of resilient material, this vacuum will slightly reduce the diameter of the tube. A strain gage mounted against the outside wall of the tube senses the diameter reduction, and activates an occlusion alarm. Repeatedly placing the disposable tubing accurately against such a strain gage is difficult and makes such systems less reliable than would be desirable.
In a similar fashion, it is known to sense the diameter of the outlet tube to monitor downstream occlusions, such as a blocked needle or obstructed tubing connection. Here, the prior art devices use stain gages to sense the diameter increase in the outlet tube caused by increased pressure which the blockage creates, so that an alarm may be activated. These sensors are subject to the same weaknesses described above.